To provide increased control of hydraulic motors, a device called a hydraulic servo was created. Frequently, the hydraulic servo is driven by an electronic stepper motor. Such hydraulic servos have three separate components: a stepper motor, a rotary valve driven by the stepper motor, and a hydraulic motor which receives hydraulic fluid from the rotary valve. In effect, the action of the stepper motor is hydraulically amplified by the hydraulic motor to provide a high-level output. Usually, the hydraulic servo is built with the three separate stepper motor, rotary valve and hydraulic motor components arranged in an end-to-end, generally coaxial relationship which results in a relatively long device. These hydraulic servos also have a relatively complex design with many long fluid passages running between the rotary valve and the hydraulic motor which increases their cost of manufacture.
It will, therefore, be appreciated that there has been a significant need for a hydraulic servo with a simpler design which is less expensive to manufacture, and with a more compact design. Further, it is desirable to produce a hydraulic servo which utilizes a fluid-powered helical actuator. Such an actuator uses a cylindrical body with an elongated rotary output shaft extending co-axially within the body. The shaft has an end portion which provides the rotary drive output. The actuator has an elongated piston sleeve disposed between the body and the shaft, with the shaft co-axially extending therethrough. Helical splines, balls in helical grooves, or rollers in helical grooves are used for transmitting torque between the piston sleeve and the body and between the piston sleeve and the shaft to produce rotation of the shaft in response to axial movement of the piston sleeve. Such an arrangement produces a relatively high torque rotary output from a simple linear input.
It is desirable to create a hydraulic servo which utilizes a helical actuator of the type just described using a rotary valve for control which can be driven manually, by a stepper motor or by other means. The resulting servo actuator should have a design which is more compact, simpler and less expensive to manufacture. The present invention fulfills these needs, and further provides other related advantages.